Modern wireless communication systems for packet based communication often use a retransmission protocol such as hybrid automatic repeat request (HARQ) on the physical layer to achieve greater reliability and robustness against the impairments of the radio channel. Long Term Evolution (LTE) and Wideband Code Division Multiple Access (WCDMA) are two examples of communication systems using HARQ. HARQ combines forward error correction (FEC) with ARQ by encoding an information containing data block, also known as transport block (TPB), in a FEC encoder and then adding cyclic redundancy check (CRC) bits or other error detection bits to the coded bits output from the FEC encoder. The coded data block is referred to as a codeword in the LTE and WCDMA systems. After reception, the data block is decoded and the CRC bits are used to check whether the decoding was successful. If the data block is received without error, an acknowledgement (ACK) is sent to the transmitter indicating successful transmission of the data block and a new data block is transmitted. On the other hand, if the data block was not decoded correctly, a negative ACK (NACK) is sent by the receiver to request a retransmission. Depending on the implementation, the transmitter may resend the same data block, or may send different data (incremental redundancy). The receiver may decode the retransmission independently or combine the retransmission with data received in the prior transmission.
Multiple antenna systems are also receiving significant attention for packet data communication systems as one way to increase data transmission rates. Multiple input, multiple output (MIMO) systems employ multiple antennas at the transmitter and receiver to transmit and receive information. The receiver can exploit the spatial dimensions of the signal at the receiver to achieve higher spectral efficiency and higher data rates without increasing bandwidth.
One transmission scheme that is often used for MIMO systems is spatial multiplexing. In a spatial multiplexing transmitter, the information symbols in the codewords output from the HARQ process are mapped to one or more spatial layers. Multiple codewords from multiple HARQ processes can be transmitted simultaneously. When multi-codeword transmission is used, the HARQ can be carried out independently on different layers. The LTE standard specifies a number of fixed codeword to layer mappings.
Some spatial multiplexing schemes support Long Delay Cyclic Delay Decoding (LDCDD) as a means of averaging the received quality (i.e., the Signal to Interference and Noise Ratio (SINR)) over all the layers. LDCDD is particularly useful in situations where the Channel Quality Information (CQI) of each layer cannot be estimated in a reliable way, e.g., in the presence of non-stationary interference or moderate to high mobility. By averaging the quality of the layers, the probability of a dip in the quality of a received codeword is reduced and the robustness of the link is increased. Averaging the quality of the layers also means the channel quality for each layer become more or less similar to the channel qualities of all the other layers
Assuming that the codeword from each HARQ process is individually mapped to a different layer or different groups of layers, an attractive decoder for multi-codewords transmission is Successive Interference Cancellation (SIC) decoder. SIC is an iterative decoding technique that estimates one codeword during each iteration. The key idea of SIC is to recursively remove from the received signal the contribution of the layers that have already been decoded. Under the realistic assumption that the previous codewords have been correctly decoded, inter-layer interference is reduced on each subsequent iteration of the decoder By doing so, the channel quality, or effective SINR, associated to each codeword increases after the interference of the previously decoded layers is removed.
If one assumes that LDCDD is employed at the transmitter, all the received codewords experience approximately the same effective SINR, which is denoted SINR0, due to the effect of the layer mixing introduced by LDCDD before the first iteration of the SIC decoder. During the first iteration of the SIC decoder, the first codeword is decoded and its contribution is subtracted from the received signal. Therefore, all the remaining codewords now experience SINR1>SINR0 due to the removal of cross layer interference from the first layer. This process is repeated for all the codewords, resulting in an increasing SINR for each codeword after interference from each previously decoded layer is removed. The effective SINR for each layer or codeword may be denoted SINR0<SINR1< . . . <SINRr where r is the number of layers.
The maximum rate of information R that can be reliably transmitted on the ith layer is a direct function of the effective SINR for the corresponding codeword, i.e., Ri=f(SINRi). Therefore, it is clear that an efficient system employing SIC and LDCDD should assign the transmit rates associated to each codeword as R0<R1< . . . <Rr.
The current assumption in the technical literature is that single codeword transmission is not suitable for SIC because the same modulation and coding scheme (MCS) would be used for each layer. Therefore, conventional wisdom in the technical literature dictates that multi-codeword transmission with a separate HARQ process for each codeword should be used when it is desired to implement SIC. The implementation of separate HARQ process may require significant upgrades to base stations that support only single rank transmissions. Further, using a separate HARQ process for each codeword increase the MAC resources and signaling overhead for multi-codeword transmission. If the data transmission rates for each codeword are correctly assigned, both the codewords have the same high probability of being decoded correctly and consequently an ACK message will be sent for both codewords. On the other hand, if the receive SINR at the first iteration has been overestimated, both codewords are likely to be incorrectly decoded and a NACK will be reported by both the HARQ processes. Thus, the increase in MAC resources and signaling overhead to implement separate HARQ processes does not in such cases appear to improve system performance.